What does "kick back" mean?

In Britain, a "kickback" is modern slang for a bribe.

To echo what others are saying, there are two meanings and both are very common in my opinion. You will see kickback (n) meaning a bribe in newspapers. To me it's a particular kind of bribe, a percentage received for directing government contracts to someone. And "kick back" (v) meaning relax is equally common.

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What does "kick back" mean?
What does "kick back" mean?
Is that a popular english slang?
Thank you.

Actually it depends on context. But in general, kickback as a phrasal noun means bribe ( the illegal money that sb pays to sb in return for a work ) and as a phrasal verb means to relax.It may have some other figurative meanings.But I don't know if it is popular.

"kickback" (informal) ---- money that someone pays secretly and *dishonestly* in return for someone's help.

It can also mean a small amount of money you get as a reward for doing something to help another make money?

I might be wrong but I think it came from "Kicking your feet up and leaning back" hence kicking back.

It means relax, basically.

Example:
"The weather was nice during spring break, so we kicked back, bathed in the sun, and enjoyed the lake to ourselves."